>>>> What compilers support any of the MMX/3Dnow!/SSE/SSE2 instruction sets>>>> (and optimize code for them)? Do you know of any published>>>> comparisons of such compilers?>>> The upcomming GCC 3.1 (The extensions was developed for x86-64 by Suse>> and AMD) and of course Intel's icc>> Is there any documentation on what GCC will be able to do in this> respect? I have heard many reports of automatic vectorization> support, and have used some (very old) patches which added automatic> use of MMX instructions and registers, but the performance increase> was minimal due to the poor code generated. Have things improved a> lot since then?

Well, the biggest gain and the focus of the project was the option
-fpmath=sse. This replaces all x87 instructions with SSE/SSE2 ones. This
gives a performance boost even without vectorization, because it makes
compiler optimizations a lot easier (more RISC like).